Sulfur dioxide oxidation catalyst beds

Several other reports have also shown the importance of effective catalyst wetting on the performance of a bench-scale trickle-bed reactor. Hartman and Coughlin37 concluded that for sulfur dioxide oxidation in qojjntercurrejQt trickle-bed reactor packed with carbon particles, the catalyst was not completely wet at low liquid flow rates (of the order of 5 x 10 4 cm s-1). Sedricks and Kenney86 found that, during catalytic hydrogenation of crotonaldehyde in a cocurrent trickle-bed reactor, liquid seeped. into dry palladium-on-alumina... 

The laboratory scale physical model of the catalytic sulfur dioxide oxidation is a 0.05 m-diameter reactor containing 3 mm-diameter pellets of catalyst over a height of 0.15 m. The bed is flushed through at 430 °C by a gas flow that contains 0.07 kmol S02/kmol total gas, 0.11 kmol 02/kmol total gas and 0.82 kmol N2/kmol total gas. The gas spatial velocity is 0.01 m/s. 

Lee, J.K. Hudgins, R.R., and Silveston, P.L., Sulfur dioxide oxidation in a periodically operated trickle-bed Comparison of activated carbon catalysts. Environ Prog., 15(4), 239-244 (1996). 

The catalytic oxidation of sulfur dioxide to sulfur trioxide can also be carried out in a fluidized bed reactor. The gas to be converted is fed in at the bottom of a fluidized bed containing the catalyst in the form of abrasion resistant beads. The whole fluidized bed can be kept at the required temperature by removing the heat of reaction with a pipe cooler. This isothermal mode of operation enables gases with a higher sulfur dioxide content to be processed and more compact plants to be built. 

At 600°C, the rate of reaction is some 30 to 50 times faster again, requiring an even smaller reactor for the same throughput, but the rate of dissociation of sulfur trioxide to sulfur dioxide becomes appreciable. The value of Kp drops to about 10, giving only about 60-65% of the sulfur as sulfur trioxide at this temperature, and the remainder as sulfur dioxide. For process purposes there is no point in considering the sulfur oxide equilibrium situation for any higher temperatures than this. With a promoted vanadium pentoxide catalyst bed at 600°C a 2-4 sec contact time is already sufficient to obtain essentially equilibrium concentrations at this temperature. 

These three reactions, catalyzed in catalytic converters, are all exothermic and ther-mod)mamically favored. Unfortunately, other energetically favored reactions are also accelerated by the mixed catalysts. All fossil fuels contain sulfur compounds, which are oxidized to sulfur dioxide during combustion. Sulfur dioxide, itself an air pollutant, undergoes further oxidation to form sulfur trioxide as it passes through the catalytic bed. 

Example 9-2 Olson and Schuler determined reaction rates for the oxidation of sulfur dioxide, using a packed bed of platinum-on-alumina catalyst pellets. A differential reactor was employed, and the partial pressures as measured from bulk-stream compositions were corrected to fluid-phase values at the catalyst surface by the methods described in Chap. 10 (see Example 10-1). The total pressure was about 790 mm Hg. 

Olson and Smith measured the rate of oxidation of sulfur dioxide with air in a differential fixed-bed reactor. The platinum catalyst was deposited on the outer surface of the cylindrical pellets. The composition and the rates of the bulk gas were known. The objective was to determine the significance of external diffusion resistance by calculating the magnitude of — C. If this difference is significant, then the values must be used in developing a rate equation for the chemical step. 

The form of the radial temperature profile in a nonadiabatic fixed-bed reactor has been observed experimentally to have a parabolic shape. Data for the oxidation of sulfur dioxide with a platinum catalyst on x -in. cylindrical pellets in a 2-in.-ID reactor are illustrated in Fig. 13-9. Results are shown for several catalyst-bed depths. The reactor wall was maintained at 197°C by a jacket of boiling glycol. This is an extreme case. The low wall temperature resulted in severe radial temperature gradients, more so than would exist in a commercial reactor, where the wall temperature would be higher. The longitudinal profiles are shown in Fig. 13-10 for the same experiment. These curves show the typical hot spots, or maxima, characteristic of exothermic reactions in a nonadiabatic reactor. The greatest increase above the reactants temperature entering the bed is at the center,... 

Example 13-5 Using the one-dimensional method, compute curves for temperature and conversion vs catalyst-bed depth for comparison with the experimental data shown in Figs. 13-10 and 13-14 for the oxidation of sulfur dioxide. The reactor consisted of a cylindrical tube, 2.06 in. ID. The superficial gas mass velocity was 350 lb/(hr)(ft ), and its inlet composition was 6.5 mole % SO2 and 93.5 mole % dry air. The catalyst was prepared from -in. cylindrical pellets of alumina and contained a surface coating of platinum (0.2 wt % of the pellet). The measured global rates in this case were not fitted to a kinetic equation, but are shown as a function of temperature and conversion in Table 13-4 and Fig. 13-13. Since a fixed inlet gas composition was used, independent variations of the partial pressures of oxygen, sulfur dioxide, and sulfur trioxide were not possible. Instead these pressures are all related to one variable, the extent of conversion. Hence the rate data shown in Table 13-4 as a function of conversion are sufficient for the calculations. The total pressure was essentially constant at 790 mm Hg. The heat of reaction was nearly constant over a considerable temperature range and was equal to — 22,700 cal/g mole of sulfur dioxide reacted. The gas mixture was predominantly air, so that its specific heat may be taken equal to that of air. The bulk density of the catalyst as packed in the reactor was 64 Ib/ft. ... 

In some cases a catalyst consists of minute particles of an active material dispersed over a less active substance called a suppori. The active material is frequently a pure metal or metal alloy. Such catalysts are called. supporred catalysts, as distinguished from unsupported catalysts. Catalysts can also have small amounts of active ingredients added called promoters, which increase their activity. Examples of supported catalysts are the packed-bed catalytic converter automobile, the platinum-on-alumina catalyst used in petroleum reforming. and the vanadium pentoxide on silica u,sed to oxidize sulfur dioxide in manufacturing sulfuric acid. On the other hand, the platinum gauze for ammonia oxidation, the promoted iron for ammonia synthesis, and the silica-alumina dehydrogenation catalyst used in butadiene manufacture typify unsupported catalysts. 

Lampert presented a catalytic partial oxidation technique for sulfur compounds that was developed by the former Engelhard (now BASF) corporation [296]. The sulfur compounds of natural gas or liquefied petroleum gas were converted into sulfur oxides at a low 0/C ratio of 0.03 in a ceramic monolith over a precious metal catalyst. These sulfur oxides were then adsorbed downstream by a fixed adsorber bed, which contained adsorption material specific to sulfur trioxide and sulfur dioxide, which could trap up to 6.7 g sulfur per 100 g adsorbent. The partial oxidation was performed at a 250 °C monolith inlet temperature, the adiabatic temperature rise in the monolith amounted to 20 K. Light sulfur compounds usually present in natural gas and liquefied petroleum gas, such as carbon oxide sulfide, ethylmercaptane, dimethyl sulfide and methylethyl sulfide, could be removed to well below the 1 ppm level. Exposure of the monolith to an air rich fuel/air mixture at temperatures exceeding 150 °C had to be avoided. The same applied for contact with fuel in the absence of air regardless of the temperature. 

Cesium is used as a catalyst in the hydrogenation of organic compounds. Some other catalysts are doped with cesium, giving an improved catalytic effect An important example is the vanadium pentoxide that catalyzes the oxidation of sulfur dioxide to sulfuric acid. The addition of cesium is expensive but profitable as it reduces the bed inlet temperature, saving energy and start-up time. It also maximizes the SO2 conversion and reduces emissions. 

Oxidation of sulfur dioxide, SO2 -I- I/2O2 SO3, is industrially carried out in fixed beds filled with V2O5 catalyst particles. Kinetic studies have indicated that the reaction rate can be described by an expression of the following kind ... 

Further development of the contact process did not rely on a better catalyst but depended on better methods to remove poisons and clean the gases produced by roasting pyrites, which, by then, had replaced sulfur as the preferr source of sulfur dioxide. In attempting to overcome the difficulty, the Mannheim process used a bed of relatively inactive iron oxide to guard the main bed of a platinum catalyst. New Jersey Zinc and the General Chemical Company in the United States built plants of this kind in 1899 and 1901, respectively. 

In the SCR process, NOX impurities are reduced with added ammonia in the presence of some residual oxygen from the furnace. The main NOX reduction reactions are shown in Table 11.5 together with some of the undesirable oxidation reactions, which can both produce sulfur trioxide and waste some of the added ammonia. Between 0.6-0.9 moles of ammonia per mole of NOX are added to limit the aimnonia shp to downstream equipment where it would deposit as sulfates. NOX conversion is therefore hmited to between 60-90%. At low NOX levels, there is little conversion to itrous oxide. Nitrous oxide formation is also inhibited by water. Gas leaving the boiler is usually at a temperature in the range 300-430°C and contains dust Dust is removed in an elee-trostatic precipitator with little heat loss before sulfur dioxide is removed as gypsum by reaction with lime. Alternatively, sulfur dioxide can also be eonvert-ed to sulfuric acid. The effluent is then vented to atmosphere. In the first power plants to be retrofitted with SCR units there were three possible loeations for the catalyst bed ... 

The space velocity was varied from 2539 to 9130 scf/hr ft3 catalyst. Carbon monoxide and ethane were at equilibrium conversion at all space velocities however, some carbon dioxide breakthrough was noticed at the higher space velocities. A bed of activated carbon and zinc oxide at 149 °C reduced the sulfur content of the feed gas from about 2 ppm to less than 0.1 ppm in order to avoid catalyst deactivation by sulfur poisoning. Subsequent tests have indicated that the catalyst is equally effective for feed gases containing up to 1 mole % benzene and 0.5 ppm sulfur (5). These are the maximum concentrations of impurities that can be present in methanation section feed gases. 

When only potassium carbonate or organic solvents are used, the effects are less important. Potassium carbonate blocks the catalyst pores, and can be removed by washing with water to restore normal performance. Methanation catalysts can be protected from poisons by installing a guard bed of zinc oxide absorbent. This will remove traces of sulfur and droplets of liquid from the carbon dioxide removal system70. 

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